The present invention generally relates to a method for manufacturing a semiconductor device, and more specifically, to a method of forming a semiconductor substrate with a SOI structure for forming a gate by an epitaxial process while forming an insulating film pattern in a bottom where a device isolation trench is formed. The invention thereby improves the electrical characteristics of the semiconductor device and increases product yield.
FIGS. 1a through 1e are cross-sectional diagrams illustrating a known method for manufacturing a semiconductor device.
After a pad oxide film 30 and a pad nitride film 40 are formed over a semiconductor substrate 10, the pad nitride film 40, the pad oxide film 30 and the semiconductor substrate 10 are etched with a device isolation mask (not shown) to form a device isolation trench 50.
A device isolation oxide film layer 60 for filling the trench 50 is formed. As design rules of the semiconductor device have been reduced, the aspect ratio of the trench 50 is relatively increased so that the process for filling an oxide film layer 60 becomes complex. As a result, the process margin is dramatically reduced, and a void is generated in the oxide film layer 60.
The pad nitride film 40 and the pad oxide film 30 are removed, and the device isolation oxide film layer 60 is planarized to form a device isolation film for defining an active region. Then, a hard mask layer is formed over the semiconductor substrate 10. A photoresist pattern 75 for defining a recess gate portion is formed over the hard mask layer. The hard mask layer is etched with the photoresist pattern 75 as an etching mask to form a hard mask pattern 70 which exposes the recess gate portion.
The semiconductor substrate 10 is etched at a predetermined thickness with the hard mask pattern 70 as an etching mask to form a recess 80. Then, a channel-impurity-ion-implanting process is performed on the active region 20.
A gate 90 is formed over the semiconductor substrate 10. Then, impurities are implanted into a region between the gates to form source/drain regions 85.
As the length of gate channels becomes shorter, the concentration of impurities implanted into the channel is required to be higher in order to prevent a punch-through phenomenon and degradation of refresh characteristics. However, when the concentration of impurities is increased, the electric field of the semiconductor substrate increases, and electrons are moved in a voltage less than a threshold voltage. The movement of electrons increases the drain voltage, and causes an interaction with the source region to lower the potential barrier of the source region and increase leakage current. Here, the gate voltage cannot control the drain voltage so that electrons between the source/drain regions are driven to the drain, which is called the punch-through phenomenon.